Solar irradiance is the power of sunlight arriving on a surface, measured in watts per square meter (W/m²). Under a clear sky with the sun high, a surface facing the sun receives close to 1,000 W/m², which is the standard test condition solar panels are rated at. Clouds, haze, the angle of the sun and the tilt of the surface all reduce it. Because panel output is nearly proportional to irradiance, it is the first number needed to judge whether a panel is performing and to estimate what a system will produce.
Components
- Direct (beam) irradiance: sunlight arriving straight from the sun’s disc. Casts sharp shadows; the part concentrators use.
- Diffuse irradiance: sunlight scattered by the sky, clouds and haze; arrives from all directions. On an overcast day it is all there is.
- Global irradiance: the sum on a horizontal surface (GHI) or on a tilted surface (POA, plane of array), plus a little reflected from the ground. Panels respond to plane-of-array global irradiance.
Typical values
| Condition | Approximate irradiance (plane facing the sun) |
|---|---|
| Extraterrestrial (above the atmosphere) | 1,361 W/m² |
| Clear noon, summer, sea level | 900 to 1,100 W/m² |
| Clear mid-morning or mid-afternoon | 600 to 800 W/m² |
| Thin cloud or haze | 400 to 700 W/m² |
| Overcast | 100 to 300 W/m² |
| Heavy overcast, rain | 50 to 100 W/m² |
| Standard test condition for panel ratings | 1,000 W/m² |
Irradiance and energy: peak sun hours
Add up irradiance over a day and you get insolation in kWh/m². Dividing by 1 kW/m² gives “peak sun hours”, the number of hours of full 1,000 W/m² sun that would deliver the same energy. A location with 5 kWh/m² per day has 5 peak sun hours, even though the sun is up for 14. Annual averages range from about 3 in cloudy northern climates to 6 or more in deserts. The solar output estimator uses peak sun hours to estimate system yield.
Measuring it
- Handheld solar power meters use a silicon photodiode or a small solar cell behind a diffuser. They respond to the same part of the spectrum a panel uses, which makes them suited to panel comparisons. Accuracy is typically ±5 to 10 W/m² plus a percentage.
- Reference cells are calibrated silicon cells matched to the panel technology; the professional field tool.
- Pyranometers use a thermopile under a glass dome and respond to the full spectrum; the meteorological reference instrument, more expensive and slower.
Hold the meter in the plane of the panel, not horizontal, and take the reading at the same moment as the panel measurement; irradiance changes by the second under broken cloud. See best solar panel testers for meters and PV testers.
Why it matters
Panel current is almost exactly proportional to irradiance; power nearly so. A 400 W panel at 500 W/m² delivers about 200 W, and that is correct behavior, not a fault. Judging a panel without an irradiance reading is guessing. See solar irradiance vs solar panel output and how to measure solar panel output.
Recommended Tools
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Frequently Asked Questions
What is a typical solar irradiance value?
Clear sky at noon in summer: 900 to 1,100 W/m² on a surface facing the sun. Overcast: 100 to 300 W/m². Just after sunrise: under 100 W/m².
What is the difference between irradiance and insolation?
Irradiance is instantaneous power (W/m²). Insolation, or irradiation, is energy accumulated over time (kWh/m² per day), often expressed as peak sun hours.
How do I measure irradiance?
With a handheld solar power meter (a calibrated photodiode or small cell) held in the plane of the panel, or a pyranometer for reference-grade measurements.
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